In the semiconductor integrated circuit industry, physical vapor deposition techniques (e.g., sputtering, evaporation) and chemical vapor deposition techniques are typically used to deposit metal onto a semiconductor wafer. However, in a recent trend, some semiconductor integrated circuit manufacturers are investigating or using electroplating techniques to deposit metal primary conductor films on semiconductor substrates. In a typical conventional electroplating process for integrated circuit applications, a metal (e.g., copper) is electrodeposited onto a semiconductor wafer. Typically, the copper layer is electrodeposited onto a substrate that has been patterned and etched to define recessed interconnect features using standard photolithographic techniques. The electrodeposited copper layer is then etched back or polished to form conductive interconnect structures.
Generally, in electroplating processes, the thickness profile of the deposited metal is controlled to be as uniform as possible. In many typical integrated circuit applications, it is advantageous for the electrodeposited metal layer to have a uniform or flat thickness profile across the substrate surface to optimize subsequent etchback or polish removal steps.
However, typical conventional electroplating techniques are susceptible to non-uniform thickness profile variations. Non-uniform thickness profiles may result from any number of causes such as the geometric size and shape of the electroplating cell, depletion effects, "hot edge" effects, and the "terminal effect".
For example, the terminal effect arises as follows. In electroplating metals onto a wafer, a conductive seed layer is typically first deposited on the wafer to facilitate electrodeposition of the metal. The seed layer is typically formed using a non-electroplating process (e.g., chemical vapor deposition, physical vapor deposition). The seed layer is needed because the wafer serves as the cathode of the electroplating cell, which requires that the wafer surface be conductive. The seed layer provides this required conductivity. Then, during the electrodeposition process, a potential is applied at the edge of the wafer.
However, because the seed layer is initially very thin, the seed layer has a significant resistance radially from the edge to the center of the wafer. This resistance contributes to a potential drop from the edge (electrical contact point) of the wafer to the center of the wafer. Thus, the potential of the seed layer is initially not uniform (i.e., tends to be more negative at the edge of the wafer) when the potential is applied. Consequently, the initial electrodeposition rate tends to be greater at the edge of the wafer relative to the interior of the wafer. As a result of this initial non-uniform deposition rate, the final electrodeposited metal layer tends to have a concave thickness profile (i.e., thicker at the edges of the wafer and thinner at the center of the wafer).
Generally, whatever the cause, non-uniformities in the final thickness profile of the electrodeposited metal are undesirable. Thus, it may be desirable to control the thickness profile of the electrodeposited metal to compensate for the non-uniformities that can arise in the electroplating process.
In other applications, it may be desirable to control the thickness of the deposited metal over the wafer to have selected non-level profiles. For example, a chemical mechanical polishing (CMP) process may be subsequently performed on the electrodeposited metal layer. Some CMP processes have non-uniform polishing rates at different locations of the wafer. Thus, it may be desirable for the metal layer to have a selected non-uniform thickness profile to compensate for the different polishing rates.
Accordingly, there is a need for an electroplating system capable of selectably controlling the thickness of the electrodeposited metal to a desired profile.